For many years, beverages, such as soft drinks and beer, have been packaged in metallic containers or cans. Originally, these metallic cans were formed of flat tin plated steel which was formed into a cylinder and sealed, by means such as soldering, welding or other sealing means, to form a side seam. A can bottom end was then seamed to the cylindrical body, the can was filled, and another can end top was seamed to complete the structure.
More recently, one-piece can bodies, formed of either aluminum or steel, have become widely used. These can bodies are formed by drawing and ironing a circular blank or slug of metal into a one-piece can body, by methods now well-known in the art. After forming this one-piece can body, the can body is filled with product and a can end, with or without an easy-opening feature thereon, is seamed to the can body to complete the structure.
Originally, both in the three-piece steel can and the two-piece aluminum or steel can, the can ends were of a somewhat greater diameter than the can body itself and thus extended axially outwardly beyond the walls of the can body. Because of this structure, when palletizing or otherwise shipping or storing filled can bodies, the effective volume of the filled can was a cylinder having the diameter of the can end, resulting in much wasted space.
To reduce the shipping and storage volume, it has now become a common practice, at least with respect to the two-piece can body, and to a somewhat lesser extent with respect to the three-piece can body, to form a reduced diameter flange portion on the can body. This practice is referred to as necking of the can body. The flange portion is necked inwardly to a reduced diameter such that the can body will accept an end of a diameter no greater than the can body diameter itself. Thus, the shipping and storage volume of the can body has been reduced to the volume of a cylinder of the diameter of the body itself.
To further reduce costs and the amount of metal in cans, double or even triple necking of the can flange region has been accomplished. In such can bodies, the final flange region diameter is reduced to accept an end having a diameter even less than the diameter of the can bodies themselves.
Necking of can bodies, however, presents problems in the design and fabrication of the can body. The necking of a can body is a diameter reduction process which supports the metal in the flange region only on the outside surface of the can body while compressing the metal. The metal is thus prevented from wrinkling only by the internal stiffness of the material itself. On the other hand, it is desired to form the can bodies having as thin a wall thickness as possible, for reduced metal usage and lighter weight and thus lower production and shipping costs. Currently, these competing forces are compromised by ironing the side wall thicknesses, in aluminum cans, to a thickness in the range of approximately 0.004 inch (0.0102 centimeters) while maintaining the thickness of the flange region in the range of approximately 0.0075 inch (0.0191 centimeters), by use of a tapered punch which carries the can body as it is ironed between the punch and ironing dies. However, such a tapered punch must be carefully machined and is subject to wear.
The metal in the flange region is, of course, additional metal which must be supplied in the metal disc or slug used to form the container, as well as providing extra weight for the container. Thus, it is a primary object of the present invention to enable reduction of the wall thickness of the necked-in flange region of a can body.
As previously mentioned, the successful necking of can bodies relies upon the stiffness of the material being formed. For a simple beam, the deflection of a metal member varies with the following formula: ##EQU1## where:
W is the width of the beam
L is the length of the beam
E is a constant for the material employed and
I is the moment of inertia.
The moment of inertia for a beam is: ##EQU2## where:
W is the width of the beam and
T is the thickness of the beam.
Thus, substituting for I, the deflection of the beam becomes: ##EQU3##
The stiffness of a beam is inversely proportional to its deflection. Thus, the stiffness of a beam varies as the cube of its thickness. For example, if the flange thickness of a can body is reduced about 8.7% from 0.0075 inch (0.0191 centimeters) to 0.0065 inch (0.0165 centimeters), the stiffness of this flange area is decreased by approximately 35%, which is also approximately equal to the percentage of increase in the tendency for this area to wrinkle. It is thus a primary purpose of the present invention to increase stiffness of the metal in the flange region of a can body by increasing the effective thickness of this flange region, while at the same time decreasing its absolute thickness. This then results in a reduction in metal usage, metal cost and weight while maintaining or increasing structural strength.